This invention relates to an apparatus for generating gas for inflating a vehicle occupant restraint in the event of a collision of the vehicle. More specifically, this invention relates to an inflator assembly which generates gas to expand an airbag in the vehicle at the instant of the collision. Still more specifically, this invention relates to such an assembly which comprises an igniter tube filled with a very fine igniter powder and an auto ignition cup in close engagement with the tube.
Many forms of inflators for air bag restraint systems are known. One form utilizes a solid fuel composition for the generation of the inflating gas and includes an annular combustion chamber bounded by inner and outer casings as disclosed in U.S. Pat. Nos. 4,296,084; 4,561,675; and 4,817,828; all of which are incorporated herein by reference. The casings are commonly made of aluminum because of its light weight. Situated in close proximity to the solid fuel, which is usually in granular form, is a primary ignition assembly which comprises an electrical squib, an igniter tube, powdered primary igniter material within the tube, and a metal foil seal at one or both ends of the tube. Upon the occurrence of a collision, the squib fires the primary igniter material whose combustion in turn initiates the burning of the solid, gas-generating fuel. Auto ignition of the primary igniter material may occur, however, in the event that the vehicle is exposed to very high temperatures (e.g., above 650.degree. F.) such as would occur when there is an electrical fire in the engine compartment. To overcome the problem arising from the degradation of the aluminum in the casings at such high temperatures and the potential injury to by-standers by fragments sent flying by a bursting of the weakened casings, an auto ignition cup filled with nitrocellulose or other material which undergoes spontaneous combustion at a much lower temperature (e.g., about 350.degree. F.) is aligned in close engagement with the igniter tube. The strength of the aluminum metal is substantially unaffected at such temperatures. The combustion of the primary igniter material is initiated by the hot gas rushing from the auto ignition cup and burning through the seal at the end of the tube facing the cup.
The practice of installing air bags on the passenger side of vehicles is growing rapidly and may soon become standard procedure. In order to provide a very large surface area for fast ignition, the primary ignition system for the passenger side inflator often uses a very finely powdered igniter material (particles as small as 20 microns) and a wire cloth or screening material is used instead of a metal foil as a seal for the igniter tube. Even when a screen having a 325 mesh size is used, however, some of the fines of the powder escape. Significant loss of powder from an igniter tube may cause a delay or possible malfunction of the ignition system. The screen is held tightly against the end of the igniter tube by a boot made from a thin membrane of an elastomeric material. To shape the boot and impart some structural integrity to it while the assembly is being press fit over the open end of the primary igniter tube, the screen is inserted into the boot before the boot is slipped onto the tube.
A resinous film about 3 mils thick has also been stretched across the central aperture of a toroidal flange to contact the end of a primary igniter tube and thus be interposed between it and an auto-ignition cup to act as a closure for the tube.